To Run on Two (or More) Legs - Why We Run: A Natural History - Bernd Heinrich

Why We Run: A Natural History - Bernd Heinrich (2002)

Chapter 12. To Run on Two (or More) Legs

Human ingenuity may make various inventions, but it will never devise any invention more beautiful, nor more simple, nor more to the purpose than Nature does, because in her inventions nothing is wanting and nothing is superfluous.

—LEONARDO DA VINCI, fifteenth century

A human runner has to make many strategic decisions. Some of those choices can be based on science, and he can look to animal models for guidance. In the same way that a painter must know the technical effects of color combinations, techniques of paint application, shading, and highlights, a runner must acknowledge physiology, the medium through which excellence is exerted. Nevertheless, although animals can reveal mechanisms, our performance, whether it is in a painting or in a race, is ultimately art because there is so much that varies.



Of course, one thing we have no control over is how many legs we have. Yet leg number varies widely between different animals, as does running speed. Since running has evolved independently in various evolutionary lines of arthropods, dinosaurs, birds, and reptiles, and in various groups of mammals, we might reasonably ask if leg number affects speed. In the fourth century B.C., Aristotle said, “If one way be better than another, that you may be sure is nature’s way.” But nature’s way is diversity. Aristotle made many original observations of nature, but he apparently did not know about historical constraints, or the compromises, which we must accommodate or adjust to.

We evolved from lobe-finned ancestors that crawled onto land with four limbs, with which we were blessed, or saddled, in subsequent evolution. Does multipedalism increase, or decrease, speed? In the arthropods, there is much variation in leg number and that allows comparisons. Millipedes, depending on species, have about a hundred or two hundred legs, and they are a study in slowness, even with most of their legs working all-out in waves, some going up and some going down at the same time. Centipedes, with only about fifty legs, are also not speedsters, except with respect to millipedes. Many of a centipede’s legs are expendable and illustrate a novel way to use legs for a getaway. One type of centipede and daddy longlegs (an arachnid) drop off legs when chased, and the loose, wiggling legs twitch and distract the attacker while the owner runs off. Spiders, with eight legs, are faster than centipedes, and they grow a new leg for every one they lose. Perhaps they need them all for web building, as well as locomotion. Insects have six legs, and some of them, like the tiger beetles I mentioned earlier, are superbly fast runners, at least on a hot day. Others are laboriously slow.

For efficiency and smoothness of stride, few insects can compare with some species of cockroaches. Considerable progress has been made in elucidating how these insects run. As revealed with high-speed cameras, the champion runner, the American cockroach, Periplaneta americana, raises three legs at a time and keeps three on the ground. The first and third on one side and the second on the other are used as a unit. The roach moves using such alternate tripods. The difference between walking and slow running is simply the rate at which successive tripod steps are taken, although when really cruising, some cockroaches do something different. They spread their wings, shift their body weight to the rear, and become bipedal by running on their hind legs. American cockroaches can sprint this way at some fifty body lengths per second. By that measure, they run about four times faster than a cheetah, the world’s fastest land animal in terms of absolute speed.


Basilisk lizard (crested water dragon)

There are great differences between species of cockroaches. David George Gordon, who has written the world’s most authoritative guide to cockroaches, points out that the German cockroach “is having a great day if it can go faster than a foot per second.” The Madagascan hissing cockroach is even slower. It is a lumbering beast that nobody has yet had the interest (or patience) to time. Arwin Provonsha, when referring to American cockroaches, says, “These puppies are born to run.” He should know. Provonsha is the curator of insect collections at Purdue University and the announcer at their annual All-American Trot, which features mostly American cockroaches. The All-American Trot, where such matters are brought under objective scrutiny, features cockroaches’ footraces on a custom-built circular track with racers coming from entomology department research stock. Such pedigreed individuals lovingly named Hot to Trot, Sewer Sam, Plain Disgusting, and the like are marked on the back with bright acrylic colors for the spectators, more than seven thousand in 1995. What induces the contestants to run is seeing the light of day; they are kept in the dark until the starter’s gun. Ultimately, the contestants are running for their lives, because throughout their more than 500-million-year evolutionary history, the cockroach that could not quickly scoot into a dark hiding place was a dead cockroach. On the other hand, if you’re big, heavily armored, and have formidable defenses, like the Madagascan hissing cockroach, then you don’t need to run fast, and probably won’t. (In the All-American Trot, the Madagascan species’ talents are harnessed not in straight-out footraces but in pulling miniature green-and-yellow John Deere tractors.) This cockroach racing stuff has its serious side, too. There is the betting, of course, and the discovery that the ultimate in cockroach running speed is achieved by bipedalism.

All of the great quadrupedal dinosaurs were probably slow or only short-distance runners, but the bipedal ones (gallimimus, compsognathus, velociraptor, and others), according to paleontological evidence, were speedsters. The ostrich, a bipedal descendant of dinosaurs, is a superbly graceful runner that cruises at 70 kilometers per hour and can keep it up for long distances. Similarly, although some present-day lizards can run well on four legs, some species—the basilisk, the crested water dragon, and others—achieve their full running speed only by rearing up onto their hind legs. By switching from the quadrupedal to the bipedal gait, the basilisk can even achieve high enough speeds to run on the water surface, hence its name “Jesus lizard.” If cockroaches and lizards achieve faster running speed by becoming bipedal, then it seems plausible that our own evolution, from semiquadrupedal ape ancestor to bipedal human, also had implications with respect to running speed. Certainly not everything, but something.

Bipedalism in mammals is associated with relatively open arid environments where long-range vision and rapid movement are both at a premium for foraging and predator avoidance. The bipedal mammals that come quickly to mind include kangaroos in Australia, springhares and early hominids in Africa, kangaroo rats and jumping mice in North America, and gerbils in Asia.

All of the bipedal animals that run fast do so by a rapid succession of long leaps, either alternating between legs or kicking off with both legs at the same time. There is considerable heavy impact of the feet striking the ground, and with that impact comes a potential loss of energy. However, mechanisms have evolved to harness some of this otherwise wasted energy. It’s in the anatomy. As the foot is depressed on landing, the heel (Achilles’) tendon is stretched, and when the foot rebounds with liftoff on the toes, the just-stretched tendon, or springing ligament, contracts and releases stored energy. Up to 40 percent of the energy absorbed by the impact is retained in this ligament, to be returned to the body during the second step. The arch of our foot also depresses and stores energy, and experiments with human feet from cadavers suggest that up to 70 percent of the energy that goes into a depressed foot arch may be returned as well (although the elasticity of muscles and tendons that gives us bounce greatly decreases with age). Obviously, running surface also makes a huge difference, a fact well known to track runners. Runners reach their greatest speeds on tracks that compress 5-8 millimeters (about the same as the compliance of the spring in the foot arch), and experimental tracks of varying stiffness can return 90 percent of the energy stored in them.

Track shoes do the same, but the compliance of the shoe has to be tuned to the running surface so that the shock of the step is not merely absorbed and the energy dissipated. Using good bouncy shoes and simultaneously running on a springy track does not add up to getting more energy back from each step than we put into it. To the contrary, energy cancels out. It’s like bouncing a ball. A rubber ball will rebound higher from a hard surface than from a surface that yields.

Given our foot design, barefoot running on the right surface can be an efficient way to race, if the soles of one’s feet are tough enough so that one can strike one’s feet solidly enough against the hard ground to generate rebound. As mentioned before, I’d already experimented in Africa, and I found out that my feet weren’t nearly tough enough. Those of Abebe Bikila were, when he ran barefoot and won the 1960 Rome Olympic marathon race in record time of 2:15:16.2. However, he subsequently ran more than 4 minutes faster in the next Olympics, at Tokyo, this time wearing running shoes. As far as I know the subsequent sub-2:10 marathoners have all been shod.

From beetles, cockroaches, ostriches, and cheetahs, evolution of greater running speed has been associated with a reduction in foot weight, achieved by reducing the number of digits, and with a lengthening of the foot and toes. This trend is best seen in the evolution of the horse. Horses run on the tip of just one single greatly strengthened and elongated toe on each foot. Ostriches also run primarily on one enlarged toe, with a second, smaller one providing lateral support. Deers and antelopes run on the tips of two toes, but the metacarpals of those two toes have become fused to form one elongated bone. A greatly elongated foot makes the leg lightest toward its terminal end, because the major muscles that power the leg are located high up near the trunk, being attached to the foot with long tendons. That arrangement not only helps to lengthen the stride, it also makes each stride energetically less costly since a light leg can be swung forward and backward faster and more easily than one weighted at the end.

Our feet are uniquely adapted for running relative to our cousins the apes’, whose feet have five digits still fully functional for grasping. Having given up easy climbing for fast running, we have toes that are now almost useless; we cannot use them for grasping, and in order to achieve top speed, we run “on our toes,” with most of the power during the step’s kickoff on the run being applied through the big toe. When we sprint, the hind part of our foot barely touches the ground, thus effectively lengthening the leg. Power comes from the front end. For all practical purposes, all of our toes could as well be fused, or our large toes could be enlarged and the others deleted, if we were uncompromisingly designed to be pure sprinters.

We don’t know which variables evolution might alter to achieve greater speed versus endurance. Undoubtedly they would be many. But if the examples of other animals is a guide, and if we were subjected to a few million years of strong selection specifically for running speed, then evolution would undoubtedly alter our feet! As I have indicated elsewhere, most women runners—even the elite—have slower running speed than men. No definitive explanation exists to account for this difference. Could foot length be a factor? Women’s feet are shorter than men’s, and in one informal survey that I did, I discovered that they were possibly even shorter than predicted on body size alone. Did males face stronger selective pressure than females to enhance running speed?

To some animals, legs can be a handicap. One very fast vertebrate land animal that I encountered on the African acacia steppe has no legs at all. Being young and foolish, I wanted the skin of this exotic animal as a trophy. I’d chased my quarry under a bush in short, dry grass. As I got close to try to slay it with my shotgun, I saw a quick movement and a set of beady coal black eyes. In almost the same instant, the creature lunged out at me. I jumped back and began to sprint. Glancing back, I could see it right at my heels. Running fast, I soon reached a clear sandy patch, at the edge of which the spitting cobra stopped, reared itself up three or four feet, and again glared at me. I turned and shot. After my sojourn in Africa, my Maine teammates joked that the snake episode had taught me how to run. Perhaps it did in the ultimate, evolutionary sense. We all learned the virtue of speed that way.

Cobras are long, thin snakes, as smooth to the touch as polished glass, all the better to slither fast. Snakes move forward much like fish do, by applying lateral force against a medium while being slippery to it. It’s basically the same principle as that used in sailing, where wind rather than muscle power provides the energy for forward momentum. Caterpillars appear to run, but they don’t do it with their legs, either (which are restricted to three very short pairs at the front). Caterpillars move forward, like seals humping along on land, by a series of posterior-anterior peristaltic waves. To imagine how they move, think of a hot dog with semiliquid contents contained by an elastic, semirigid shell. Internal muscles contract in series from tail to head, and as a contraction wave passes any one point of the body, that portion is lifted off the ground and is telescoped forward for a “step” as the caterpillar extends. Speed is largely a function of “stride” frequency, not stride length. The caterpillars’ legs hold and anchor, but at this point in evolution they no longer have anything to do with their original role in powering the locomotion. Their legs show us what they once did. The ancestors of snakes also had legs that ultimately became useless, if not an impediment to locomotion, yet some of them still have vestigial leg elements internally.

In humans, morphology also still gives us clues of our ancient history. As Charles Darwin stated in On the Origin of Species, “Organs now of trifling importance have probably in some cases been of high importance to an early progenitor, and, after having been slowly perfected at a former period, have been transmitted to existing species in nearly the same state, although now of very slight use.” An animal’s musculoskeletal system provides clues to selective pressures that have acted on the organism. Similarly, our nervous systems and our basic behavioral tendencies are just as much products of natural selection as are our muscles and bones.

Morphologically and behaviorally, we reflect our past. For us to run well requires not only an efficient bipedal running form, but also elastic Achilles’ tendons, strong big toes, and perhaps even more than anything else, special psychological tendencies. I will propose a hypothesis of what those psychological tendencies might be and how they could have arisen in our prehistory as apes on the African savanna.